Colorectal cancer is a devastating disease, ranking as the third most commonly diagnosed US cancer and second leading cause of cancer death. There have been only incremental advancements in the treatment of metastatic colorectal cancer so an urgent unmet medical need exists to design and develop new drugs. The central challenge has been to validate novel drug targets and to generate new therapies that disrupt these molecular targets. While protein kinases are among the most popular molecular cancer targets, the 22 oncogenic human protein tyrosine phosphatases that are implicated in cancer have not received suitable attention and none have therapeutically attractive inhibitors. Phosphatases clearly have a non-redundant role in cancer as well as participate in adaptive responses to oncogenes and therapies. Validating a protein tyrosine phosphatase as a molecular target for colorectal cancer and helping to develop small molecules as leads for the treatment of colorectal cancer would have an enormous impact. There is growing evidence that Protein Tyrosine Phosphatase 4A3 (PTP4A3; a/k/a PRL-3) is involved in cancer progression and metastasis. Further target validation and information about small molecule inhibitor interactions are needed. This innovative proposal brings together an experienced multidisciplinary team that will use x-ray crystallography to understand small molecule inhibitor interactions and exploit a recently generated animal model and cells to address these challenges. If successful, this proposal will generate new reagents and information that will enhance our understanding of the role of PTP4A3 in cancer biology and facilitate the design and development of an entirely new class of anticancer agents. In this interdisciplinary approach, we propose two innovative and complementary Specific Aims. Specific Aim 1: Determining the interactions of small molecule ligands with PTP4A3. Specific Aim 2: Define the functional role of PTP4A3.